Minimally invasive and non-invasive therapeutic ultrasound treatments can be used to ablate, necrotize, and/or otherwise damage tissue. High intensity focused ultrasound (“HIFU”), for example, is used to thermally or mechanically damage tissue. HIFU thermal treatments increase the temperature of tissue at a focal region such that the tissue quickly forms a thermally coagulated treatment volume. HIFU treatments can also cause mechanical disruption of tissue with well-demarcated regions of mechanically emulsified treatment volumes that have little remaining cellular integrity. For certain medical applications, tissue emulsification may be more favorable than thermal damage because it produces liquefied volumes that can be more easily removed or absorbed by the body than thermally coagulated solid volumes.
HIFU treatments can utilize a sequence of pulses, rather than continuous-wave HIFU exposures, to reduce undesirable thermal effects on the surrounding tissue. In histotripsy exposures, for example, HIFU sources operate with low duty cycles (e.g., 1%), use relatively short pulses (e.g., 10-20 microseconds), and deliver high pulse average intensities of up to 40 kW/cm2 to form bubbles that mechanically disrupt tissue. Histotripsy techniques, for example, can induce cavitation by delivering pulses of high peak negative pressures that are significantly higher than the tensile strength of the tissue. The repetition of such pulses can increase the area of tissue affected by cavitation to create a “cavitation cloud” that emulsifies the tissue. Cavitation, however, is generally stochastic in nature, making cavitation-based HIFU treatments somewhat unpredictable and difficult to reproduce. For example, cavitation activity can stop unexpectedly during the course of the treatment, resulting in the extinction of the cavitation cloud and termination of the desired tissue emulsification. Very high peak negative pressures of about −20 MPa are required to initiate and maintain the cavitation cloud. To reach these peak negative pressure levels, large aperture transducers with high focusing angles and high power output capabilities are necessary. Therefore, there is a need to enhance the reliability, predictability, and consistency of mechanical disruption of tissue damage (e.g., emulsification), while operating at lower pressure levels and still limiting thermal coagulation of the target tissue and the surrounding tissue.